The present invention relates in general to an active antenna for the reception of long, medium, short and ultra-short wave broadcasts and being mounted on an electrically heated rear window of a motor vehicle. The heated rear window is equipped with a set of heating elements connected via bus bars to direct current power connections and including also connections to an antenna amplifier.
In designing antennas of this kind it is necessary to obtain an optimum reception both in the long, medium and short wave range and in the ultra-short wave range and also to prevent a pick-up of high frequency interferences, for example from the network of the motor vehicle.
An antenna of this kind is known for example from the German application P No. 26 50 044. In this known antenna the set of heating elements serves simultaneously as an antenna element for the reception both of the short, medium and short waves and for ultra-short wave signals. In this design, there is a problem particularly in the supply of direct current for the heating elements. Especially in the long, medium and short wave range in which the set of heating elements due to relatively low frequencies constitutes a high impedence antenna element, the supply of large direct currents necessary for heating the window causes a considerable damping of the received signals. In the prior art solution, the heating direct currents are supplied through a bifilar choke or reactance coil whereby the choke is connected for high frequency signals parallel to the antenna element. Especially at low frequencies however it is not possible to set the reactance of the choke to a sufficiently high value to match the broad band of the L-M-S wave range so that the parallel connection of the choke to the antenna do not markedly impair the reception of the signal. In the ultra-short wave range in which the set of heating elements represents a substantially low impedance antenna element, the choking of the direct current supply can be substantially simpler and without expensive technological measures.
Accordingly, in contrast to the L-M-S range the reception of ultra-short wave signals by the antenna according to P No. 26 50 044 is satisfactory. Another disadvantage of this prior art antenna however is interference from coupling to the receiver input, particularly at lower frequencies. The high frequency interferences are generated by electrical systems in the motor vehicle for example due to ignition and fuel injection control pulses. Inasmuch as the antenna according to P No. 26 50 044 has its antenna element connected both to the input of a receiver and in the case of a turned-on rear window heating, also to an electrical power supply in which is disturbed by high frequency interferences, there are necessary highly effective filtering means particularly for the relatively low frequency L-M-S wave range in order to avoid disturbances at the input of the receiver. The technical expenditures for the provision of such filtering devices, due to high intensity of heating currents (up to about 30 A) are considerable.
A similar antenna is known also from the application P No. 23 60 672. This known antenna is possessed with similar disadvantages.